There has heretofore been a method which, by redundantly configuring routers (a master router and a backup router) based on a protocol such as a VRRP (Virtual Router Redundancy Protocol), guarantees communication between terminals within a subnet and a terminal in another network, using the backup router, even in the event that a failure occurs in the master router.
Also, there being a plurality of layer 2 switches within the subnet, for example, in the event that a connection between the layer 2 switches has been severed, a terminal within the subnet appears which cannot communicate with the terminal in the other network by way of the master router. Assuming this kind of case, JP-A-2005-192027 discloses a network control system which, by causing the backup router to make a transition to the master router, guarantees the communication between the terminal within the subnet, which cannot communicate with the terminal in the other network by way of the original master router, and the terminal in the other network, by using the backup router which has made the transition to the master router.
Furthermore, there being a plurality of layer 2 switches within the subnet, for example, in the event that a connection between the layer 2 switches has been severed, a combination of terminals appears which cannot communicate with each other within the subnet. Assuming this kind of case, JP-A-2002-232448 discloses a network system which uses the redundantly configured router to enable the communication between the terminals within the subnet via a backbone network. However, with the technology disclosed therein, an efficient medium access control (MAC) address relearning cannot be expected during the severance and a fixing thereof within the subnet. Specifically, as it is necessary for each of the terminals within the subnet, during the severance, to relearn MAC addresses of the terminals other than itself, and furthermore, during the fixing too, to relearn the MAC addresses of the terminals other than itself in a condition before the severance, there are many processes carried out by the terminals, and a load on the network is large, preventing the efficient MAC address relearning.